1. Field of the Invention
The present invention relates to a PON (Passive Optical Network) system, and more particularly to a GE-PON (Gigabit Ethernet frame Passive Optical Network) system and a MAC (Media Access Control) method for the same.
2. Description of the Related Art
PON systems typically are configured to have a passive distributor or a WDM (Wave Division Multiplexing) element deployed between a subscriber access node, such as a FTTH (Fiber To The Home) or FTTC (Fiber To The Curb), and a NT (Network Termination) node. In this type of configuration, all the nodes are distributed in a bus or tree structure, thereby forming an overall topology. A typical PON system, known as an ATM (Asynchronous Transfer Mode)-PON, is well known in the art, and its MAC technique are described in detail in the ITU-T (International Telecommunication Union-T) G.983.1.
The MAC protocols used in the conventional ATM PON system are described in U.S. Pat. No. 5,978,374 issued on Nov. 2, 1999, entitled “PROTOCOL FOR DATA COMMUNICATION OVER A POINT-TO-MULTIPOINT PASSIVE OPTICAL NETWORK”, and in Korean Patent Laid-open No. 1999-70901 published on Sep. 15, 1999, entitled “METHOD FOR IMPLEMENTING ASYNCHRONOUS PASSIVE OPTICAL NETWORK MEDIA ACCESS CONTROL PROTOCOL”, which are incorporated herein by reference. Briefly, the MAC of the ATM-PON system will hereinafter be described.
FIGS. 1a˜1b illustrate a block diagram of the ATM-PON system recommended by ITU-T G.983. The ATM-PON system shown in FIG. 1a or 1b is located on a tree structure route and has one OLT (Optical Line Termination) 10, which plays an important role in providing each subscriber associated with the access network with information. The OLT 10 has a tree topology and is connected to an ODN (Optical Distribution Network) 12. The ODN 12 distributes downstream data frames received from the OLT 10 as shown in FIG. 1a, or multiplexes upstream data frames, then transmits them to the OLT 10 as shown in FIG. 1b. The ODN 12 is connected to at least two ONUs (Optical Network Units) 14i (where, i=a, b and c; and a, b and c represent a natural number). The ONUs 14i are configured to receive the downstream data frames from the ODN 12 and transmit them to a plurality of users 16i (where, i=a, b and c; and a, b and c represent a natural number), as shown in FIG. 1a. The ONUs 14i use the output data from the users 16i as upstream data frames and transmit the upstream data frames to the ODN 12, as shown in FIG. 1b. Note that in FIGS. 1a˜1b, the numerals 16i (i.e., 16a to 16c) denote users and may also represent an NT (Network Termination) or a variety of access network terminating units usable in a PON.
The conventional ATM-PON system shown in FIGS. 1a˜1b performs upstream transmission or downstream transmission in the form of data frames each of which has a predetermined number of mutually changed ATM cells. Each ATM cell has a size of 53 bytes. In the tree-shaped PON structure shown in FIGS. 1a˜1b, the OLT 10 properly inserts downstream cells to be distributed into each of the ONUs 14i into downstream frames. In case of upstream transmissions, the OLT 10 gains access to the data transmitted from the ONUs 14i according to a TDM (Time Division Multiplexing) protocol. In this case, the ODN 12 connected between the OLT 10 and the ONUs 14i functions as a passive component. The OLT 10 prevents data in the ODN 12 from colliding with one another by means of a virtual distance correction using a ranging algorithm. Further, in the case where the OLT 10 transmits downstream data to the ONUs 14i, an encryption key and an OAM (Operation, Administration and Maintenance) message are interchanged between the OLT 10 and the ONUs 14i in such a way that data confidentiality between the OLT 10 and the ONUs 14i can be ensured. To this end, the upstream/downstream frames have a dedicated ATM cell used for a message interchange at predetermined intervals or have a corresponding data field in a general ATM cell.
As described above, the ATM-PON system configures upstream/downstream frames based on a predetermined-sized ATM cell and performs a MAC protocol using a TDMA in the upstream transmission due to a point-to-multipoint tree structure. As a result, the band allocation algorithm of ONUs becomes complicated.
In the meantime, following the current trend of rapidly growing Internet technology, the demand for higher bandwidths is increasing. Therefore, many manufacturers have developed a PON system using a gigabit Ethernet instead of an ATM technique in order to accommodate the end-to-end transmission. The gigabit Ethernet has a relatively low production cost and wider bandwidth, whereas the ATM technique has a very high production cost and a limited bandwidth, and requires the Internet protocol packet segmentation.
A representative example known as a GE-PON system, using an Ethernet frame instead of the ATM in a PON structure of an access network, is made available by Alloptics Company. The GE-PON system manufactured by the Alloptics allocates a timeslot of a fixed size of 2 ms to each ONU. In this manner, the GE-PON system of Alloptics simplifies a band allocation algorithm because timeslots allocated to each ONU is fixed to a predetermined size. This GE-PON system, however, has drawbacks in that the bandwidths are wasted when there is no upstream/downstream data.
Thus, there is a need in the art for improved GE-PON systems that overcome this shortcoming of the prior art.